The present invention relates to a constant velocity universal joint for use in a power transmission of automobiles or various types of industrial machines and, more particularly, to a tripod type constant velocity universal joint.
As a tripod type constant velocity universal joint, for example, known is the one with a configuration shown in FIGS. 6 and 7. The constant velocity universal joint comprises an outer joint member 21 having three axial track grooves 22 formed in the inner circumference portion thereof and having respective axial roller guideways 22a on both sides of each of the track grooves 22, and a tripod member 24 having three radially protruding trunnions 25, said tripod member 24 rotatably mounting a roller 20 to a cylindrical outer circumference surface of each of the trunnions 25 via needle rollers 26 respectively. Each roller 20 is accommodated in and mated with the roller guideways 22a of each of the track grooves 22 of the outer joint member 21 respectively. Each of the rollers 20 rolls on the roller guideways 22a while rotating about the center of axis of each of the trunnions 25, thereby smoothly guiding relative axial displacement and angular displacement between the outer joint member 21 and the tripod member 24. Simultaneously, smoothly guided is an axial displacement of each of trunnions 25, with respect to the roller guideways 22a, accompanying a change in the rotational phase during the transmission of rotational torque between the outer joint member 21 and the tripod member 24 at a predetermined operating angle.
In practice, however, as shown in FIGS. 8 and 9, when the outer joint member 21 and the tripod member 24 transmit rotational torque while taking at an operating angle .theta., as the trunnions 25 being inclined, each of the rollers 20 and the roller guideways 22a are angularly disposed. Consequently, this prevents smooth rolling of each of the rollers 20. That is, the roller 20 is to rotate in the direction shown with the arrow a (about the axis of the trunnion 25) in FIG. 8. On the other hand, since the roller guideway 22a has an arcuate cross section and extends in parallel to the axis of the outer joint member 21, the roller 20 is to roll to move axially on the roller guideway 22a in such a manner as to gauge the same. Accordingly, sliding occurs at the contact portion between the outer circumference surface of the roller 20 and the roller guideway 22a to cause frictional resistance (sliding resistance) to increase, contributing to an increase in the induced thrust. The induced thrust contributes to inducing vibration at the joint portion. For example, in the power transmission of an automobile, there is such a problem that a large amount of induced thrust at the joint portion provides discomfort to passengers. This is because the thrust is transmitted to the automobile body via the power transmission path and is amplified, and produces resonance with the vibration of the automobile body, thus resulting in increased noise.
In view of the foregoing circumstances, in order to eliminate the angular disposition between a roller and a roller guideway and hereby reduce the induced thrust, a constant velocity universal joint with a configuration shown in FIGS. 10 and 11 has been proposed (Japanese Patent Publication No.Hei 3 (1991) -1529, etc.). This has contributed to reducing vibration and noise. This constant velocity universal joint has a roller which is mounted to the trunnion 25 of the tripod member 24, said roller comprising two types of rollers of an outer roller 23 and an inner roller 27. This allows an inclination between the outer roller 23 and the trunnion 25 (and also inner roller 27) (an inclination mechanism). The outer roller 23 is in contact with and rolls on the roller guideways 22a. The inner roller 27 has a spherical outer circumference surface 27b to engage the cylindrical inner circumference surface 23a of the outer roller 23. The inner circumference surface of the inner roller 27 engages the cylindrical outer circumference surface of the trunnion 25 via the needle rollers 26.
As shown in FIG. 12, when the outer joint member 21 and the tripod member 24 transmit rotational torque while taking an operating angle .theta., the inner roller 27 is inclined with respect to the roller guideways 22a as the trunnion 25 inclines. However, the outer roller 23 is allowed an inclination with respect to the trunnion 25 and the inner roller 27, thereby allowed for maintaining parallel attitude with respect to and rolling on the roller guideways 22a. Consequently, smooth rolling of the outer roller 23 is ensured, the sliding resistance with the roller guideways 22a is reduced, thus the induced thrust is suppressed.
As described in the foregoing, the constant velocity universal joint shown in FIGS. 10 and 11 has reduced induced thrust compared with conventional joints, however, there was a limit in further reducing the induced thrust. As the reason for this, the attitude of the outer roller is believed to be unstable with respect to the roller guideway although the constant velocity universal joint shown in FIGS. 10 and 11 can maintain the parallel attitude of the outer roller to some extent by the inclination mechanism. This instability may be caused by a slight inclination of the outer roller in the longitudinal sectional direction of the outer joint member (the sectional direction including the axis of the outer joint member). Alternatively, a slight inclination of the outer roller in the cross sectional direction of the outer joint member (the sectional direction perpendicular to the axis of the outer joint member) may cause the instability. The inclinations may be resulted from the effect of the frictional force at the contact portion of the outer and inner roller or the imbalance of joint loading acting on the contact portions of the outer and inner roller, and on those of the outer roller and the roller guideways.
Based on the foregoing circumstances, the applicant had already filed a patent application for a constant velocity universal joint with a configuration exemplified in FIG. 5 in order to further reduce the induced thrust (Japanese Patent Laid-Open Publication No.Hei 9 (1997) -14280). In the drawing, an inner roller 3' fits rotatably over the outer circumference of a trunnion 2a' of a tripod member 2' via a plurality of needle rollers 7'. Movement of the inner roller 3' in the direction of axis Z of the trunnion 2a' is limited by means of a retainer ring 8' (and a snap ring 9') attached to the distal end of the trunnion 2a' and a washer 10' attached to the proximal end of the trunnion 2a'. In fact, there is a slight axial clearance .delta.' between the needle roller 7' and the inner roller 3', and between the retainer ring 8' and washer 10' (the clearance .delta.' is made far larger than actually is for purposes of illustration). The inner circumference surface 3a' of the inner roller 3' is a cylindrically shaped one and the outer circumference surface 3b' is a spherically shaped convex one. The generating line of the outer circumference surface 3b' is a circular arc of a radius of R1 with the center thereof at point O1' which is shifted outward by a predetermined amount from the center of radius O2' of the inner roller 3'.
The outer roller 4' fits rotatably over the outer circumference surface 3b' of the inner roller 3'. In the example shown in the figure, the inner circumference surface 4a' of the outer roller 4' has a conical shape whose diameter decreases toward the distal end of the trunnion 2a'. The angle of inclination .alpha.' of the inner circumference surface 4a' (see FIG. 4) takes a small value, for example, on the order of 0.1.degree. to 3.degree., however, the degree of inclination is fairly exaggerated in the drawing. The generating line of an outer circumference surface 4b' is a circular arc of a radius of R3 with the center thereof at point O3'. The outer circumference surface 4b' is in angular contact with a roller guideway 1a' of an outer joint member 1' at two points, p' and q'. Centerline L1 which includes the center O3' of the outer circumference surface 4b' of the outer roller 4' and which is orthogonal to the axis Z of the trunnion 2a' is designed to be coincident with centerline L2. The centerline L2 includes the center O1' of the outer circumference surface 3b' of the inner roller 3' and is orthogonal to the axis Z of the trunnion 2a'.
As shown in FIG. 4, the inner circumference surface 4a' of the outer roller 4' has a conical shape whose diameter gradually decreases toward the distal end of the trunnion (upward in the figure). This causes a loading component force F to be generated toward the distal end of the trunnion at the contact position S' between the surface 4a' and the outer circumference surface 3b' of the inner roller 3'. The loading component force F acts in such a manner as to push the outer roller 4' up toward the distal end of the trunnion, reducing the contact pressure at portion B (see FIG. 5(a)) of the roller guideway 1a' on the non-loading side. In addition, a force F' is generated toward the proximal end of the trunnion (downward in the figure) as a reactive force against the loading component force F at the contact position S'. The reactive force F' acts in such a manner as to push the inner roller 3' down toward the proximal end of the trunnion, suppressing the axial movement of the inner roller 3' and needle rollers 7' with respect to the trunnion 2a'. As shown in FIG. 5(b), this allows the inner roller 3' and needle rollers 7' to be always pushed against the washer 10' on the lower side, thus suppressing a variation in the contact position S' resulting from the axial clearance .delta.'.
The constant velocity universal joint with the aforementioned configuration contributes to further reducing the induced thrust resulting from the decrease in contact pressure at portion B of the roller guideway 1a' on the non-loading side in conjunction with the stability at the contact position S'. However, on the course of many experiments, the induced thrust was found to exceed a target value in some test samples. The cause of the excess has not yet been clarified, however, may be concluded as follows.
That is, when the joint transmits rotational torque while taking an operating angle, accompanying a phase change in the rotational direction of the trunnion 2a', the contact position S' is to move reciprocally within a predetermined region from a reference point at which the operating angle is 0.degree. (the position shown in FIG. 4) toward the proximal end of the trunnion (downward in the figure) and toward the distal end of the trunnion (upward in the figure) where the amount of movement from a position of an operating angle of 0.degree. toward the proximal end of the trunnion is greater than that toward the distal end of the trunnion. However, in the constant velocity universal joint with the aforementioned configuration, the contact position S' at an operating angle of 0.degree. is shifted by .DELTA.H' toward the distal end of the trunnion (upward in the figure) from the center line L1 (which divides the distance between angular contact points p' and q' into equal parts) of the outer circumference surface 4b' of the outer roller 4'. This results from a geometrical relationship since the inner circumference surface 4a' of the outer roller 4' has an angle of inclination .alpha.', but .DELTA.H' is actually very small, unlike shown in the figure, since the angle of inclination .alpha.' is very small. In addition, the loading component force F occurs toward the distal end of the trunnion at the contact position S'. Therefore, the joint loading is localized at the contact point p' on the distal end of the trunnion when the joint rotates taking an operating angle .theta. and thus a variation in the loading balance is generated between the contact points p' and q'. The variation in the loading balance between the contact points p' and q' causes the inclination (drift) of the outer roller 4' to increase, which is considered to contribute to increasing the induced thrust.
Furthermore, in a constant velocity universal joint of this type, the guideway clearance between the outer circumference surface of the outer roller and the roller guideway is generally made as small as possible to reduce the backlash of the joint. Thus, a slight inclination of the outer roller 4' causes the roller guideway 1a' on the non-loading side to be in comparatively strong contact with the outer circumference surface 4b' of the outer roller 4'. This is considered to result in a comparatively high contact pressure at the roller guideway 1a, on the loading side. As such, the relationship between the degree of inclination of the outer roller with respect to the roller guideway and the clearance of the guideway causes the surface pressure of the contact portion of the both to increase, the sliding resistance to increase, and the smooth rolling of the outer roller to be interfered. This is considered to contribute to increasing the induced thrust.